The Arms (Prohibited Firearms, Magazines and Parts) Amendment bill was reported back from select committee last night and is expected to be in force by Friday.
The bill would ban MSSAs and assault rifles, with some exemptions for hunting, pest-control, collectors' items, and heirlooms and mementos.
Gun owners will have an amnesty until September 30 - which the Government can extend - to hand in their soon-to-be-illegal semi-automatics.
Nash conceded there was no way of knowing how many MSSAs and assault rifles will remain in circulation because there is no way of knowing how many there are.
He supported the recommended changes that the committee made to the bill, including extending exemptions to cover pest-control on private and non-conservation land, and for heirlooms and mementos - under strict conditions.
But the committee did not support extending exemptions for shooting competitions, or for seven-round cartridges for semi-automatic shotguns.
Pistol NZ had pleaded with the committee to allow semi-automatics under strict conditions for the thousands of law-abiding competitive shooters, and Nash said the Government could look at the issue again in the second tranche of reform.
"We just thought that opened a number of loopholes, but maybe again we'll look at that at stage two."
National MP Judith Collins, a former Police Minister, also pushed for competitive shooting to be reconsidered.
"Our view is we could look at the rule around pistol shooting as one of the ways to extend that to other shooting competitions."
Collins added that the buyback scheme should include gun dealers who will not be reimbursed by their suppliers, and maybe gun parts as well.
"That buyback scheme will need to be thorough, it will need to be generous, and one that most gun owners will support."
The Government had initially ruled out dealers, but Nash said that was being reconsidered.
The National Party wanted to include Firearms Prohibition Orders in the bill to give greater powers for police to search and seize illegal guns, including from gangs.
Nash said that could also be considered in the second tranche of reforms.
"If we're gong to put those in, it needs a level of public consultation ... Rushing through FPOs is not the way to do that."
He defended the bill's clause that allowed the Government, through Order in Council, to change the definition of what firearms should be illegal without the support of Parliament, saying such powers already existed in the Arms Act.